Final Report Summary - MECHWNTSIGNALS (Mechanisms of Wnt Signaling Initiation) Wnt proteins control embryonic development in all multicellular organisms, including humans. In adults, these proteins also sustain the vital supply of stem cells that replenish various body tissues (e.g. bone, nervous system and skin repair). The interaction of Wnt proteins with cell surface receptors on receiving cells can result in a variety of cellular responses, including cell proliferation and cell fate decisions, as well as organized cell movements and establishment of tissue polarity. Deregulated Wnt signalling due to mutations is strongly linked to cancer and degenerative disease. Together, the Wnt signalling pathway holds promise for therapeutical intervention in the fields of cancer, stem cell biology and regenerative medicine.With the MechWNTsignals ERC StG project, I aimed to gain understanding of the molecular mechanisms by which cells interpret and process Wnt signals received at their cell surface and how dysregulation of these events by mutations leads to cancer. By uncovering molecular details of how protein traffic, complex formation and activity direct cellular decisions we aim to provide novel clues to modulate Wnt-mediated cellular responses. Over the course of the project, my team has made a number of discoveries, highlighted below:1) We elucidated the first known step in WNT signalling, the interaction between the FZD receptor and its cytoplasmic effector Dvl, by employing sophisticated peptide libraries. Our finding that multiple parts of FZD make contact with a previously overlooked domain in Dvl significantly alters the current view on how these proteins communicate to steer activation of β-catenin-mediated transcription. 2) We identified the deubiquitinating enzyme CYLD as a novel negative WNT regulator. Our findings imply that Dvl ubiquitination positively drives WNT signal transmission, and link dysregulated WNT signalling to skin tumorigenesis in CYLD-mutant cylindromatosis patients. 3) We discovered a novel, negative feedback loop by which intestinal stem cells downregulate their FZD receptors to inhibit tumour growth. Our findings implicate RNF43 as a novel tumor suppressor and suggest that inhibitors of receptor activation may be useful in the treatment of tumors with RNF43 mutations. 4) The tumour suppressor Axin1 coordinates formation of a complex that downregulates β-catenin to prevent target gene activation. We discovered that Axin employs large, natively disordered regions in performing its activity. Our results support a model in which the disordered nature of these critical scaffolding regions in Axin1 facilitates dynamic interactions with a kinase and its substrate, which in turn act upon each other.5) We employed a biochemical two-dimensional approach to monitor formation of native protein complexes that operate in the Wnt cascade. Our results argue that during Wnt stimulation as well as in APC-mutant cancer cells, a late step in β-catenin degradation is inhibited to allow for target gene transcription.
